1. Field of the Invention
The present invention relates to photocatalyst excitation apparatuses.
2. Description of the Related Art
Conventional photocatalyst materials showing catalytic functions by irradiation of light include titanium dioxide, tungsten oxide, vanadium oxide, zirconium oxide, zinc oxide, zinc sulfide, and tin oxide. Recently, titanium dioxide (TiO.sub.2) has attracted attention due to high oxidative decomposition ability, antifouling properties, and hydrophobicity thereof.
Photocatalyst excitation apparatuses using such photocatalysts have various structures depending on the use. In general, as shown in FIG. 6, a TiO.sub.2 photocatalyst layer 82 is formed on a substrate 80 composed of a tile, glass or plastic and is irradiated with excitation light 84 for the photocatalyst, such as ultraviolet light, from the upper side.
When the TiO.sub.2 photocatalyst layer 82 is irradiated with the excitation light 84, electrons are excited by the photoelectric effect so that electrons and holes are generated and migrate to the surface of the TiO.sub.2 photocatalyst layer 82. Electrons reduce oxygen in air to form superoxide ions (O.sub.2.sup.-), whereas holes degrade water adsorbed on the surface to form hydroxyl radicals (.OH). The superoxide ions and hydroxyl radicals are called activated oxygen species and show strong oxidizing effects.
When organic contaminants adhere to the TiO.sub.2 photocatalyst layer 82, superoxide ions deprive the organic compound of carbon whereas hydroxyl radicals deprive the organic compound of hydrogen to decompose the organic compound. The decomposed carbon and hydrogen are oxidized to form carbon dioxide and water. Oxidative decomposition of and antifouling properties to organic substances are thereby shown.
In the above conventional photocatalyst excitation apparatus, solar light containing ultraviolet light or ultraviolet light emitted from an artificial light source is used as the excitation light 84 which is incident on the photocatalyst layer 82.
Since a light source separately placed at the exterior of the photocatalyst excitation apparatus is used in such a case, the excitation light 84 may be absorbed or scattered in media such as air and moisture which are present between the light source and the TiO.sub.2 photocatalyst layer 82. Thus, the excitation light 84 for the photocatalyst may be attenuated when it reaches the TiO.sub.2 photocatalyst layer 82. Accordingly, the optical power from the light source is not effectively used.
When solar light is used as the excitation light 84, the luminous power of the solar light significantly depends on the weather out of doors, and the solar light is shaded or diminished indoors. Thus, the TiO.sub.2 photocatalyst layer 82 does not stably work as the photocatalyst.
When a nondirectional light source such as a fluorescent lamp is used as the light source of the excitation light 84, some part of the light is scattered and is not incident on the TiO.sub.2 photocatalyst layer 82. Thus, the optical power of the light source is not effectively used. When a highly directional light source such as a semiconductor laser or a light emitting diode (LED) is used, mismatch of the irradiating zone of the light source and the position of the TiO.sub.2 photocatalyst layer 82 causes dissipation of the light from the light source to regions other than the TiO.sub.2 photocatalyst layer 82. Thus, the optical power of the light source also cannot be effectively used.
When an ultraviolet light source is used as the light source for the excitation light, which is radiated towards regions other than the TiO.sub.2 photocatalyst layer 82, may reach the eyes and skin. Thus, the effects on human bodies, particularly the possibility of melanoma carcinogenesis concerns. When the photocatalyst excitation apparatus is used in products, in which people view for a long time, such as a Braun-tube screen of a television set and a windshield of an automobile, the above hazards will be severe problems.
When an artificial light source is used as the light source for the excitation light, a space is required for independently placing the light source. Thus, the possibility of the use of the photocatalyst excitation apparatus is limited and the esthetics thereof may be deteriorated.
In general, the activity of the catalyst increases as the thickness of the TiO.sub.2 photocatalyst layer 82 increases. When light with a wavelength which has large absorption in the TiO.sub.2 photocatalyst layer 82 is used as the excitation light 84, the light is absorbed in a shallow region near the surface of the TiO.sub.2 photocatalyst layer 82, and thus uniform excitation is not achieved in the deep region. When light with a wavelength which has small absorption in the TiO.sub.2 photocatalyst layer 82 is used as the excitation light 84, the TiO.sub.2 photocatalyst layer 82 is uniformly excited from the surface to the deep region, but the excitation efficiency is not high due to low light absorption. Accordingly, even if the thickness of the TiO.sub.2 photocatalyst layer 82 is sufficiently increased to enhance the activity, the increased thickness is not effectively used in any case.